Make a Lattice or Tree diagram and count the results that give a prime.
In this context, I considered '1' as a prime, I know it is not a true prime number. I did forget to put 2 in my prime number list, my mistake..
...(4/6)(1/6)(1/6) + (1/6)(3/6)(1/6)+ (1/6)(1/6)(3/6)
...4/216 + 3/216 + 3/216
...10/216 (reduce)
...5/108
That's how I got 5/108